Why exercise programs must increase in difficulty and duration over time to remain effective

Because, as you exercise at a certain level, your body grows, stretches, and grows accustom to the level you're working at. As your body gets stronger and adapts, you have to keep challenging it or else it'll just get weaker. When you're straining your body with difficult exercises your muscles rip and strain, but they regrow and replenish even stronger. If you're doing an exercise your body can already handle with ease, your muscles will not strain, thus they will not get stronger. Hope I helped.